Thermoplastic vulcanizates (TPVs) are produced via dynamic vulcanization of immiscible blends of a rubber and a thermoplastic, i.e., the selective crosslinking of the rubber while melt mixing with the thermoplastic. The resulting TPVs comprise cross-linked rubber particles dispersed in a thermoplastic matrix, which have both elasticity and melt processability. The majority of commercial TPVs are based on blends of ethylene/propylene/diene monomer (EPDM)-type interpolymers with polypropylene (PP), and crosslinked with phenolic or peroxide curing agents. EPDMs typically used in commercial TPV formulations have very high molecular weights; e.g., Mooney viscosity (ML (1+4@125° C.)) greater than, or equal to, 200. To improve processability, extender oil is often added to the EPDM polymer to reduce the apparent viscosity during production. The resulting bale form of EPDM has disadvantages in handling. For example, the bale form of EPDM requires an additional grinding step before being fed into an extruder.
Oil-extended EPDM interpolymers have limited compounding flexibility, because such interpolymers contain a fixed level and/or certain type of oil in a pre-compounding state. Typically regular non-clear paraffinic oil is used as the extender oil. The regular paraffinic oil contains aromatic and polar fractions, which may be undesirable in the compounded formulation. For example, such oils are generally dark colored and are undesirable for producing white or colored TPVs. The level of oil may also restrict compounding options. For example, a TPV formulation with very low oil concentration is not achievable with an oil-extended EPDM because most oil extended EPDM has 75 PHR or more oil in it.
PCT Publication WO2009/123609A1 discloses a process for preparing a thermoplastic vulcanizate, the process comprising charging a reactor with an olefinic copolymer rubber, where the olefinic copolymer rubber is character by a multimodal molecular weight, an average branching index of greater than 0.8, includes less than 10 parts by weight oil per 100 parts by weight rubber, includes less than 1 parts by weight non-rubber particulate, per 100 parts by weight rubber, and is in the form of granules having a particle size less than 8 mm. The reactor is charged, contemporaneously, or sequentially with respect to the rubber, with a thermoplastic resin, an oil, and a cure system, and the rubber, the thermoplastic resin, the oil, and the cure system are melt mixed, and the rubber is dynamically vulcanized.
U.S. Patent Publication No. 20080033089 discloses a thermoplastic vulcanizate composition comprising a dynamically-cured rubber; from about 20 to about 300 parts by weight of a thermoplastic resin per 100 parts by weight rubber and from about 30 to about 250 parts by weight additional oil per 100 parts by weight rubber. The rubber comprises a multimodal polymer composition cured with a curing agent, the multimodal polymer composition comprising 45 to 75 wt % of a first polymer fraction and 25 to 55 wt % of a second polymer fraction, each comprising ethylene, a C3-C10 α-olefin, and a non-conjugated diene. The polymer fractions have been polymerized using a Ziegler-Natta catalyst system. The first polymer fraction has a Mooney viscosity of, greater than, or equal to, about 150 ML(1+4@125° C.), and the second polymer fraction has a Mooney viscosity of about 20 ML to about 120 ML; and about 10 phr to about 50 phr of an extender oil. This publication also discloses a method of producing the thermoplastic vulcanizate.
Despite the variety of EPDM-based formulations currently known and used in TPVs, there is a need for a formulation formed from an EPDM in the form of free flowing pellets, as opposed to highly oil extended bales, and which provides the same, or better, balance of properties as compared to the properties of conventional TPV formulations prepared using bale form, ultra high molecular weight EPDMs. There is further a need for such TPV formulations which provide ease in handling and formulation flexibility. Various embodiments of the following invention meet some and/or all of these needs.